The State Department said Tuesday it has revoked U.S. travel visas for a number of individuals after a Nigerian man with a visa to travel to the United States tried to bring down a jetliner over Detroit December 25. Officials say the number of revocations is not large and that the United States wants to remain a welcoming country for legitimate business and tourist travelers.

The State Department says it has revoked the U.S. visas of a number of individuals including the Nigerian jailed for the attempted Christmas day bombing, Umar Farouk Abdulmutallab, as part of an inter-agency security upgrade spurred by the Detroit incident.

President Obama cited a failure of the security system after it was revealed that the jailed Nigerian's father had gone to the U.S. Embassy in Abuja before the incident to voice concern about his son's apparent radicalization and visit to Yemen.

An account of the embassy conversation was relayed to counter-terrorism officials in Washington but the son's U.S. travel visa was not rescinded.

At a news briefing, Assistant Secretary of State for Public Affairs P.J. Crowley said that as a result of the administration's ongoing security review, multiple individuals have had U.S. visas revoked and others added to U.S. no-fly and terrorism watch lists.

Crowley said the upgrade was global in scope but said it was not fruitful to provide a specific number of those affected.

But he stressed that the United States wants to remain a country welcoming to visitors, and said the new policy by no means precludes legitimate visits by persons from countries like Yemen, Nigeria and 12 others for which intensified security for U.S.-bound travelers is now required.

"We want to have these people come to the United States. It's in our interests. It's in our foreign policy interests. This interaction between the American people and people of other countries is actually part of the process by which, ultimately, we will defeat and mitigate political extremism," he said. "So we're not closing our doors to the United States. Far from it. We welcome people coming to the United States. We're going to make sure that the process by which people come here, and travel here, is as safe as it can be," he added.

Crowley acknowledged the State Department is receiving complaints - what he termed feed-back - from the 14 governments affected by the tighter screening procedures but did not elaborate.

He said one reason why the U.S. embassy in Yemen's capital Sanaa reopened Tuesday after a security scare was to resume processing visa applications from Yemeni travelers.

The spokesman said about 1,700 U.S. travel visas had been revoked for security reasons since the September 2001 terrorist attacks on the United States. He said the fact the number of new revocations since December 25 is relatively small reflects the basic strength of screening procedures.